The Maximum Likelihood Detector which is also known as the Optimum Noncoherent Detector (or correlation detector) for detecting frequency shifted keyed (FSK) signals in an additive white gaussian noise channel is well known. The performance of a correlation detector can also be achieved with other detector architectures such as a Matched Filter or a Fast Fourier Transform (FFT). These detectors have the ability to achieve a significant sensitivity improvement (i.e., 4 dB) over that of discriminators for M-level orthogonal signaling (i.e. FLEX.TM.). However, when 2-level FLEX.TM. or POCSAG signals with Non-Standard deviations occur on the channel, severe degradations can occur. For 2 level FLEXT.TM. or POCSAG signaling, only the outer correlator bins of the correlator detector as shown in FIG. 1 are necessary for detection. However the 2 inner correlators are often used as well (see FIG. 2) while only passing the most significant bit (MSB) of the symbol decision to the decoder.
Older systems such as those being used for POCSAG or Golay may have transmitters with poor deviation accuracy. Even though the POCSAG protocol states that the nominal deviations are +/-4500 Hz, measurements in the field have shown that they may vary anywhere from +/-2600 Hz to +/-5400 Hz as shown in FIG. 3. These same (Pocsag) systems are often used to initially transmit 2-level FLEX.TM. signals until capacity warrants them moving to 4-level. Even using all 4 correlators as shown in FIG. 2 would result in severely degraded performance if the deviations of the transmitter where at or near +/-3200 Hz. What is needed is a method which does not allow the degradation to occur when 2-level signaling is on the channel, but rather minimizes or removes the degradation allowing for the maximum improvement offered by use of the correlation detector in spite of the possibility of Non-Standard deviations existing on the channel.